The present invention relates to a method for the partial desalination of water using a combination of a weakly acid cation exchanger in free acid form and a basic anion exchanger in hydrogen carbonate (bicarbonate) form, both exchangers being present in aqueous suspensions, and the subsequent regeneration of the charged ion exchanger materials.
As a result of increasing demands for water, more and more water supply systems are forced to use ground water or surface water which, although hygienically acceptable, has too high a salt content. The dissolved salts in these waters are mainly calcium and magnesium compounds whose concentrations are generally determined by regional geochemical conditions. The "Atlas zur Trinkwasserqualitat der Bundesrepublik Deutschland", in translation, "Atlas of Drinking Water Quality of the Federal Republic of Germany" shows that hard groundwaters, that is, those having relatively high concentrations of Ca.sup.++ and/or Mg.sup.++, occur more frequently in Southern Germany. Water from lime or dolomite layers have higher carbonate hardnesses, but there also are areas where the groundwater contains much sulfate and the total salt content is at about 1000 mg/l, for example, in Central Franconia. In areas with intensive agricultural usage and correspondingly heavy fertilization, the groundwater often contains nitrate ions in concentrations up to 250 mg/l, which may be damaging to human health.
To avoid health damage, the Drinking Water Regulations of the Federal Republic of Germany prescribe that the sulfate content must be a maximum of 250 mg/l and the nitrate content no more than 90 mg/l. According to an EG (European Community) Guideline, this latter limit value will be lowered in the future to 50 mg/l, which will force numerous waterworks to take suitable processing measures to achieve this lower limit.
It is the general opinion that the total salt content of drinking water should not exceed 500 mg/l. A limitation on water hardness and neutral salt content is therefore desirable in many cases, and is particularly desirable from the point of view of chemical corrosion. When hard water is used, corrosion phenomena can be expected in zinc-coated pipelines and will be enhanced by the presence of neutral salt anions in higher concentrations. Also for reasons of corrosion protection, it is necessary, when mixing waters from different sources, to effect partial softening with matching of the carbonate hardnesses.
Partial desalination is also of significance for industrial use of water. In many cases cooling water must be partially softened. In industrial plants, large quantities of salt often enter the waste water which could be reused if it were possible to inexpensively and effectively reduce the salt content.
Weakly acid cation exchanger resins contain carboxyl groups as functional components. Analogously to the dissociation behavior of weak acids, for example, organic acids, these functional components are only weakly dissociated. Such weakly acid exchangers therefore have only a limited operating range, namely, an operating pH range of &gt;4 to 14, and are capable only of dissociating salts of weak acids, for example, salts of carbonic acid. In water processing, weakly acid cation exchanger resins are used mostly for decarbonization (softening), i.e. to remove a quantity of cations equivalent to the hydrogen carbonate (bicarbonate) concentration of the water. Due to the selectivity of exchanger resins for multivalent cations, such a weakly acid cation exchanger absorbs predominantly calcium and magnesium ions.
During the regeneration, these absorbed cations must be displaced again by hydrogen ions. Customarily this is done with nitric acid or sulfuric acid, in a concentration which avoids prepcipitation of calcium sulfate dihydrate.
Weakly acid exchangers have a distinctly strong affinity to H.sup.+ ions so that the regenerating acid, in contradistinction to the regeneration of strongly acid exchanger resins, needs to be neither particularly pure nor particularly concentrated. Therefore, the weakly acid exchangers can also be regenerated with weak acid. The use of carbonic acid as a regeneration agent for weakly acid cation exchange resins was proposed for the first time in 1953 by Gray and Crosby in U.S. Pat. No. 2,656,245.
Kunin and Vassiliou, in Industrial and Engin. Chem. Product Research and Development, Volume 2 (1963), No. 1, pages 1 to 3, describe the use of CO.sub.2 for the regeneration of sodium charged carboxylic cation exchanger resins under pressures up to 300 psi. However, the regeneration effect must be supported by extraction of the (alkali) NaHCO.sub.3 solution which is formed during the regeneration. A similar method is disclosed in U.S. Pat. No. 3,691,109 to Larsen where the discharged NaHCO.sub.3 solution from the regeneration of a weakly acid cation exchanger is further processed by degassing to form a degassed solution which can then be used to regenerate weakly basic anion exchange resins.
Berger-Wittmar and Sontheimer, in an article entitled "Regeneration schwach saurer lonenaustauscher mit Kohlendioxid," appearing in Vom Wasser, Vol. 50, 1979, pages 297 to 329, describe the effectiveness of the regeneration of Ca.sup.++, Mg.sup.++, Na.sup.+ and K.sup.+ charged weakly acid cation exchanger resins. It was found there that bivalent calcium and magnesium ions are bound more strongly to the exchanger resins than monovalent cations. In order to realize a somewhat satisfactory efficiency, the article discloses that higher CO.sub.2 pressures are required.
German Offenlegungsschrift No. 2,714,297 discloses a process for regenerating weakly acid ion exchanger resins by carbonic acid to precipitate calcium carbonate. In this process, the weakly acid cation exchange resin is regenerated in a fluidized bed under increased pressure. In order to accelerate the calcium carbonate precipitation, crystallization seeds in the form of powdered, finely crystalline CaCO.sub.3 are added from the start. This addition of CaCO.sub.3, however, reduces the efficiency of the regeneration since it increases the pH to such an extent that only little useful capacity can be generated.
Charged anion exchange resins can be partially converted to the bicarbonate ion charged form with the aid of CO.sub.2. However, this conversion is successful only if the regenerating solution has a pH at which the bicarbonate ions produced by the introduction of CO.sub.2 have a sufficient concentration. With CO.sub.2 alone, the effect is only minimal. In the process according to German Offenlegungsschrift No. 2,851,135, corresponding to U.S. Pat. No. 4,299,922, this difficulty has been overcome by adding a solid calcium compound to provide a favorable pH. The addition of such a compound, e.g. in the form of CaCO.sub.3, however, has drawbacks.
The following processes are considered to be state of the art with respect to partial desalination:
(a) Partial desalination with decarbonization with weakly acid ion exchangers. The exchangers are introduced in the free acid form and remove a quantity of bivalent metal cations equivalent to the hydrogen carbonate concentration of the water. The carbonate hardness is converted to degasifiable carbonic acid while sulfates, nitrates and chlorides remain uninfluenced. See Dorfner, Ionenaustauscher, in translation, Ion Exchangers, 3rd Edition, published by De Gruyter, Berlin, 1970, pages 170-173.
(b) Full desalination of a partial stream of the untreated water with the aid of strongly acid cation exchangers in H.sup.+ form and strongly basic anion exchangers in OH.sup.- form, and subsequent mixing of the resulting fully desalinated stream with the untreated water. The cation exchangers are regenerated with hydrochloric acid or sulfuric acid, and the anion exchangers are regenerated with soda liquor. (Usual technique in partial demineralization). The cation and anion exchangers can also be present together in the form of a mixed bed.
(c) The DESAL process. In the Desal process, a weakly basic anion exchange resin in HCO.sub.3.sup.- form is used in a first column and a weakly acid cation exchange resin is used in a second column which is connected in series with the first column. The water to be treated is passed through the weakly basic anion exchange resin to initially convert neutral salts to bicarbonates. The water then passes through the series connected second column in which the weakly acid cation exchanger then removes all cations and converts the bicarbonates to carbonic acid. In a two-bed Desal process, the resulting carbonic acid is extracted as CO.sub.2 by passage through a decarbonator. In a three-bed Desal process, the resulting carbonic acid is passed into a third column containing a weakly basic anion exchange resin in the free-base form to convert the resin in the third column to the bicarbonate form. See Dorfner, supra, pages 186 to 188, and Epstein et al, "Desalination of Brackish Waters By Ion Exchange," Ion Exchange and Membranes, 1973 Vol. 1, pp. 159-170.
The Desal process has the following drawbacks:
The weakly basic resin in bicarbonate form in the first column converts the neutral salts, preferably NaCl, to bicarbonates. The series-connected weakly acid exchanger, which is capable of dissociating only the salts of weak acids (e.g. salts of carbonic acid), then removes a quantity of cations equivalent to the bicarbonate concentration. The elimination of neutral salt anions and neutral salt cations is thus coupled stoichiometrically.
For regeneration in the two-bed Desal system, the charged weakly basic resin in the first column is initially converted into the free base form with NH.sub.3 and is then brought into the bicarbonate form with CO.sub.2. The weakly acid exchanger is regenerated with sulfuric acid. For regeneration in the three-bed Desal process, the weakly basic anion exchanger in the first column is converted into the free base form with NH.sub.3 and the weakly basic anion exchanger in the third column is converted by the introduction of carbon dioxide to a major portion in the bicarbonate form and to a minor portion in the hydroxyl form. The weakly acid exchanger is regenerated with sulfuric acid. Thereafter, the three columns are again ready for the next desalination process, which then takes place in the opposite direction, i.e. the water to be treated is now charged into the third column.
(d) The LARSEN process disclosed in U.S. Pat. No. 3,691,109.
In the Larsen process, the neutral salts are converted to bicarbonates by passage through a weakly basic anion exchange resin present in free base form. The effluent from the weakly basic anion exchange resin is then passed through a lime softening bed where the bivalent calcium and magnesium ions are precipitated with the aid of lime (CaO and Ca(OH).sub.2) as CaCO.sub.3 or Mg(OH).sub.2, respectively. The effluent from the lime softening bed is then passed to a weakly acid cation exchanger in the free acid form which removes the remaining monovalent cations (predominantly Na.sup.+). Because of the intermediate precipitation with lime in the lime softening bed, the actual ion exchange is not stoichmetrically coupled.
For regeneration, a solution of carbon dioxide in water, that is, carbonic acid, such as obtained by employing CO.sub.2 by-products of refinery operations, is first conducted over the cation exchanger to displace sodium ions. The resulting effluent from this step, which contains sodium bicarbonate and free carbonic acid, is degassed, i.e. the carbon dioxide is removed, and the solution obtained in this way is used to regenerate the weakly basic anion exchanger to the free base form.
The Larsen process is limited exclusively to the regeneration of Na charged cation exchangers. The regeneration of Ca charged cation exchangers is expressly excluded from the process according to the Larsen patent because suitable chemical conditions cannot be obtained due to CaCO.sub.3 precipitations the pH of Ca(HCO.sub.3).sub.2 solutions is lower than the pH of NaHCO.sub.3 solutions) for the regeneration of the cation exchanger.
(e) The SIROTHERM process. This process uses exchangers which contain weakly acid as well as weakly basic groups. In the desalination phase, both functional components are present in the free acid or base form, respectively. See, Dorfner, supra, page 188.
The desalination in the Sirotherm process is therefore also stoichiometrically coupled with respect to the removal of the neutral salt ions. The desalination is used preferably for removal of NaCl from brackfish water.
Regeneration in the Sirotherm process is effected with hot raw water, the higher degree of dissociation of H.sub.2 O molecules being used to make available the H.sup.+ and OH.sup.- ions.
The last three processes, the Desal process, the Larsen process and the Sirotherm process, are intended for the elimination of common salt (sodium chloride), i.e. generally 1-1-valent salts. The aim in each case is a substantial desalination of the raw water down to a small residual concentration. The occurrence of bivalent ions, such as Ca.sup.++, and SO.sub.4.sup.--, makes regeneration much more difficult in the Sirotherm and Larsen processes, and the desired goal can no longer be reached. In the Desal process, the regeneration is performed with different chemicals so that this difficulty is avoided.
All three processes use a weakly basic resin as the anion exchanger. This resin is able to exchange anions only from acid to neutral solutions. Alkali waters convert the functional groups of weakly basic resins to the free base form in which they are no longer able to absorb anions. Ground water is usually lightly alkali with a pH of approximately 7 to 8.5, so that these processes cannot be used there for that reason as well.